Mechanical and chemical-mechanical planarizing processes (collectively "CMP") are used in the manufacturing of microelectronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic-device substrate assemblies. FIG. 1 schematically illustrates a planarizing machine 10 with a platen or table 20, a carrier assembly 30, a polishing pad 40 positioned on the table 20, and a planarizing fluid 44 on the polishing pad 40. The planarizing machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 for supporting the polishing pad 40. In many planarizing machines, a drive assembly 26 rotates (arrow A) and/or reciprocates (arrow B) the platen 20 to move the polishing pad 40 during planarization.
The carrier assembly 30 controls and protects a substrate 12 during planarization. The carrier assembly 30 typically has a substrate holder 32 with a pad 34 that holds the substrate 12 via suction. A drive assembly 36 of the carrier assembly 30 typically rotates and/or translates the substrate holder 32 (arrows C and D, respectively). The substrate holder 32, however, may be a weighted, free-floating disk (not shown) that slides over the polishing pad 40.
The combination of the polishing pad 40 and the planarizing fluid 44 generally define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate 12. The polishing pad 40 may be a conventional polishing pad composed of a polymeric material (e.g., polyurethane) without abrasive particles, or it may be an abrasive polishing pad with abrasive particles fixedly bonded to a suspension material. In a typical application, the planarizing fluid 44 may be a CMP slurry with abrasive particles and chemicals for use with a conventional nonabrasive polishing pad. In other applications, the planarizing fluid 44 may be a chemical solution without abrasive particles for use with an abrasive polishing pad.
To planarize the substrate 12 with the planarizing machine 10, the carrier assembly 30 presses the substrate 12 against a planarizing surface 42 of the polishing pad 40 in the presence of the planarizing fluid 44. The platen 20 and/or the substrate holder 32 then move relative to one another to translate the substrate 12 across the planarizing surface 42. As a result, the abrasive particles and/or the chemicals in the planarizing medium remove material from the surface of the substrate 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other features, many substrates develop large "step heights" that create a highly topographic surface across the substrate. Yet, as the density of integrated circuits increases, it is necessary to have a planar substrate surface at several stages of processing the substrate because non-uniform substrate surfaces significantly increase the difficulty of forming sub-micron features. For example, it is difficult to accurately focus photo-patterns to within tolerances approaching 0.1 .mu.m on non-uniform substrate surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical substrate surface into a highly uniform, planar substrate surface.
In the competitive semiconductor industry, it is also highly desirable to have a high yield in CMP processes by producing a uniformly planar surface at a desired endpoint on a substrate assembly as quickly as possible. For example, when a conductive layer on a substrate is under-planarized in the formation of contacts or interconnects, many of these components may not be electrically isolated from one another because undesirable portions of the conductive layer may remain on the substrate over a dielectric layer. Additionally, when a substrate is over planarized, components below the desired endpoint may be damaged or completely destroyed. Thus, to provide a high yield of operable microelectronic devices, CMP processing should quickly remove material until the desired endpoint is reached.
The planarity of the finished substrates and the yield of CMP processing is a function of several factors, one of which is the rate at which material is removed from the substrate assembly (the "polishing rate"). Although it is desirable to have a high polishing rate to reduce the duration of each planarizing cycle, the polishing rate should be uniform across the substrate to produce a uniformly planar surface. The polishing rate should also be consistent to accurately endpoint CMP processing at a desired elevation in the substrate assembly. The polishing rate, therefore, should be controlled to provide accurate, reproducible results.
In conventional CMP processes, the polishing rate may not be uniform across the substrate assembly or consistent from one planarizing cycle to another. The polishing rate itself is influenced by several factors. One factor that influences the polishing rate is the distribution of planarizing fluid 44 between the substrate assembly 12 and the planarizing surface of the polishing pad 40. The distribution of the planarizing fluid 44 may not be uniform across the surface of the substrate assembly 12 because the leading edge of the substrate assembly 12 relative to the motion between the substrate assembly 12 and the planarizing surface 42 wipes a significant portion of the planarizing fluid 44 off of the polishing pad 40 before the planarizing fluid 44 can contact the other areas of the substrate assembly. The non-uniform distribution of planarizing fluid 44 under the substrate 12 can cause certain areas of the substrate assembly 12 to have a higher polishing rate than other areas because they have more contact with the chemicals and/or abrasive particles in the planarizing fluid. The surface of the substrate assembly 12 may accordingly not be uniformly planar, and in extreme cases, some devices may be damaged or destroyed by CMP processing.
The polishing rate may also vary from one substrate assembly to another, or even across a particular substrate, because the composition of the planarizing fluid 44 may vary. The chemicals added to the planarizing fluid 44 may degrade over time causing one batch of planarizing fluid 44 to have a different polishing rate than another batch of planarizing fluid 44. Additionally, many components in the planarizing fluid 44 settle in a liquid solution, and thus the concentration of chemicals of a particular batch of planarizing fluid 44 may also vary. As a result of the changes in the composition of the planarizing fluid 44, the polishing rate of a particular substrate assembly 12 may change making it difficult to uniformly planarize the substrate assembly 12 and to stop the planarization at a desired endpoint.
One technique for controlling the polishing rate to more uniformly remove material from the substrate assemblies is to provide better "transportation" of the planarizing fluid under the substrate assemblies. For example, the polishing pad may have grooves or wells to hold some of the planarizing solution under the substrate assemblies. In other applications, the planarizing fluid is pumped through the pad. Although providing transportation of the planarizing fluid enhances the distribution of the planarizing fluid under substrate and produces a more uniform polishing rate, many CMP applications still suffer from non-uniform and inconsistent polishing rates because of the variations in the composition of the planarizing fluid itself from one batch of fluid to another. Thus, CMP processing may not provide sufficiently planar surfaces or an adequate yield of operable devices.